Laser Ablation Technique for Electrical Contact to Buried Electrically Conducting Layers in Diamond

ABSTRACT

A method of laser ablation for electrical contact to a buried electrically conducting layer in diamond comprising polishing a single crystal diamond substrate having a first carbon surface, implanting the diamond with a beam of 180 KeV followed by 150 KeV C +  ions at fluencies of 4×10 15  ions/cm 2  and 5×10 15  ions/cm 2  respectively, forming an electrically conducting carbon layer beneath the first carbon surface, and ablating the single crystal diamond which lies between the electrically conducting layer and the first carbon surface.

REFERENCE TO RELATED APPLICATION

This application is a non-provisional of, and claims priority to and thebenefits of, U.S. Provisional Patent Application 61/613,049 filed onMar. 20, 2012, the entirety of which is hereby incorporated byreference.

BACKGROUND

This disclosure provides for a method for a laser ablation technique forelectrical contact to a buried electrically conducting layer in diamond.Furthermore this disclosure provides for a device that has a singlecrystal diamond substrate with a buried electrically conducting layerbetween two openings formed by the laser ablation technique and havingelectrical contacts and wherein the resistance measured between theopenings is dominated by the buried electrically conducting layer and onthe order of about 1 kΩ.

This disclosure demonstrates a facile method to make low-resistanceelectrical contact to a buried conducting layer in diamond.

Previous methods for establishing contact to a buried, electricallyconducting layer in an otherwise insulating diamond have not overcomethe obstacles and have do not produced the superior results of thetechnique disclosed herein.

One example of a previous method is multiple ion implants into diamond(Prins, 1985). This method for establishing electrical contact to theburied implant layer involves performing a series of ion implants over arange of energies to create a damaged diamond region which extends fromthe surface to the implant layer. Depending on the depth of the buriedlayer, this method requires many ion implants. Performing many ionimplants is a costly and time-consuming procedure requiring highlyspecialized equipment and skilled personnel to provide maintenance andoperation. Furthermore, using the multiple implant method, theelectrical contacts to the buried layer must be established before theburied layer is formed by ion implantation.

A second example of a previous method is a laser damage column (Prawer,1992). This alternative method establishes electrical contact to theburied layer by creating a column of damaged, electrically conductingdiamond which extends from the diamond surface to the buried layer usingpulsed focused laser irradiation coupled to a conventional microscopewith a 50× objective. If the buried layer is slightly opaque, the laserpulse is selectively absorbed in the buried layer. The laser melts thediamond and this melt front propagates to the surface. Upon cooling,highly conductive columns extend from the surface to the buried layer. Adisadvantage of this method is that the columns of damaged, electricallyconducting material create relatively large contact resistances to theburied layer.

A third example of a previous method is ion implant over gold beads(Olivero, 2009). This alternative is to scan an ion beam along a linearpath which terminates at a semi-spherical mask characterized by anon-uniform thickness profile (Au bead made with ball bonder). As thebeam scan progresses towards the center of the mask, incident ions crossan increasing thickness of masking material, thus progressively reducingtheir range in the diamond layer. After removing the mask, the heavilydamaged, electrically conducting diamond can be connected with thesurface and the implant layer establishing electrical contact. Adisadvantage of this method is that irregularities in the semi-sphericalmask create discontinuities in the electrically conducting column suchthat robust electrical contact between the diamond surface and theburied layer is not reliable.

SUMMARY OF DISCLOSURE Description

This disclosure provides for a method for a laser ablation technique forelectrical contact to a buried electrically conducting layer in diamond.Furthermore this disclosure provides for a device that has a singlecrystal diamond substrate with a buried electrically conducting layerbetween two openings formed by the laser ablation technique and havingelectrical contacts and wherein the resistance measured between theopenings is dominated by the buried electrically conducting layer and onthe order of about 1 kΩ.

Example

Single crystal diamond substrates were prepared by mechanical polishing.A series of treatments to remove contaminants was also performed. Thisprocedure reduces surface roughness to a small fraction of a nanometer,ensuring that the buried layer has a uniform depth and thickness. Atroom temperature the diamond was implanted, at 7 degrees to the C(100)axis to avoid ion channeling, with a beam of 180 keV followed by 150 keVC⁺ ions at fluences of 4×10¹⁵ ions/cm² and 5×10¹⁵ ions/cm² respectively.During the implant process the near-surface region of the diamondremains relatively undamaged while a thin (˜200 nm), heavily damaged,electrically conducting carbon layer is formed beginning at about 50 nmbeneath the diamond surface.

DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings.

FIG. 1 is a process schematic for fabricating electrical contacts to aburied implant layer in diamond. Part (a) illustrates the polisheddiamond in cross section; Part (b) illustrates the implant process; Part(c) illustrates the fabrication of electrical contacts. Part (c) showshow the implant layer acts as an ablation stop providing access to theimplant layer for electrical contact. Part (d) top view of the diamondshowing the fabricated electrical contacts as black squares. The insetin part (d) shows the milled contact pad and the coordinate system usedby the laser micro-machining system to raster the sample stage formilling the electrical contacts. Figure is not to scale.

FIG. 2 illustrates an optical micrograph of a milled opening in thediamond skin by laser micro-machining. The milled area provideselectrical contact to the buried implant layer as shown in FIG. 2.Ablated material from laser micro-machining is also observed.

FIG. 3 illustrates: a) Surface profilometry of laser ablated electricalcontact pad and b) Ion implant damage for (1.5 MeV, C⁺) as a function ofdepth using SRIM. The dashed line is a guide for the eye highlightingthe agreement between surface profilometry taken over a electricalcontact pad fabricated using the laser ablation technique and ionimplantation damage as a function of depth simulated with SRIM. Thisdemonstrates that with tuning of the laser parameters, only the diamondabove the implant layer is removed.

FIG. 4 illustrates I-V measurements between two contacts composed of:damaged carbon produced by the laser machining process; Cr evaporatedover the laser damaged carbon; Cr evaporated over focused ion beamfabricated contacts. The inset shows an I-V measurement taken between alaser machined contact pad capped with Cr and a tungsten point contacton the diamond surface.

DETAILED DESCRIPTION OF THE INVENTION

This disclosure provides for a method for a laser ablation technique forelectrical contact to a buried electrically conducting layer in diamond.Furthermore this disclosure provides for a device that has a singlecrystal diamond substrate with a buried electrically conducting layerbetween two openings formed by the laser ablation technique and havingelectrical contacts and wherein the resistance measured between theopenings is dominated by the buried electrically conducting layer and onthe order of about 1 kΩ.

This disclosure demonstrates a facile method to make low-resistanceelectrical contact to a buried conducting layer in diamond.

The invention disclosed herein ablates the electrically insulatingdiamond skin which lies over the electrically conducting buried layerproviding access for electrical contact. We use laser micro-machining toopen an electrical contact through the diamond skin exposing the buriedlayer.

Optical transmission microscopy of heavily implanted diamond revealsthat the buried layer is opaque so that the laser pulse is primarilyabsorbed in the implant layer. Machining (laser ablation) of the diamondis performed using a diode pumped, tripled Nd:YAG laser (355 nm)(E-Series Laser Micromachining System, Oxford Lasers Ltd., Didcot, UK.)focused 0.5 mm above the diamond surface. Laser output pulses of 17 μJare defocused to an ˜9 μm diameter beam spot. The laser pulse durationis nominally 35 ns with a pulse frequency of 10 kHz. Two 300×300 μmsquare openings spaced 2.45 mm apart are milled into the diamond skin(see FIG. 1).

The milling routine translates the sample stage in a serpentine patternat a linear speed of 5 mm/sec to mill out the opening (see the inset ofFIG. 1 part (d)). Surface profile measurements find that the milledregions are at the same depth as predicted using a simulation after asingle pass of the laser (see FIG. 3).

After laser milling, a layer of opaque, electrically conducting, carboncovers the surface of the milled region (FIG. 2) and makes a reliableelectrical contact to the conducting buried layer. A robust and facileelectrical contact is made by metal evaporation over an area covering,but slightly larger than, the carburized laser-milled regions.

Current-voltage (I-V) measurements (Keithley 2400 SMU) determined theresistance from one contact pad to the other (see FIG. 4), from onecontact pad to the diamond surface, and from separated points on thediamond surface. These measurements find linear I-V characteristicsbetween contacts.

In contrast, measurements carried out between similarly separatedtungsten point probe contacts on the diamond surface, or between acontact pad and tungsten point probe contact on the diamond surface,show dramatically higher resistance (>2 GΩ) at small applied fields, andnon-linear behavior for larger bias voltage (inset of FIG. 4).

The results from the I-V characterization find that the resistancemeasured between the contact pads is dominated by the electricallyconducting buried layer (˜1 kΩ).

Several advantages of this invention are that the contact resistance tothe buried layer is reduced. This laser ablation process is a facile,timesaving method to establish electrical contact to a buried,electrically conducting layer over a wide range of depths in anotherwise insulating diamond. For example, removal of 1 μm of diamond(see FIG. 3).

As discussed earlier, multiple ion implants into diamond, laser damagecolumns, and ion implant over gold beads are previously used methods.However, this invention described herein provides for superior results,for example, reduced contact resistance to the buried layer.Furthermore, this laser ablation process is a facile, timesaving methodto establish electrical contact to a buried, electrically conductinglayer in an otherwise insulating diamond.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been illustrated and/or described with respect toonly one of several implementations, such feature may be combined withone or more other features of the other implementations as may bedesired and advantageous for any given or particular application. Also,to the extent that the terms “including”, “includes”, “having”, “has”,“with”, or variants thereof are used in the detailed description and/orin the claims, such terms are intended to be inclusive in a mannersimilar to the term “comprising”.

What we claim is:
 1. A method of laser ablation for electrical contactto a buried electrically conducting layer in diamond comprising:polishing a single crystal diamond substrate having a first carbonsurface; implanting the diamond with a beam of 180 KeV followed by 150KeV C⁺ ions at fluencies of 4×10¹⁵ ions/cm² and 5×10¹⁵ ions/cm²respectively; forming an electrically conducting carbon layer beneaththe first carbon surface; and ablating the single crystal diamond whichlies between the electrically conducting layer and the first carbonsurface.
 2. The method of claim 1 wherein the step of ablating uses adiode pumped tripled Nd:YAG laser at 355 nm and wherein the laser pulseduration is about 35 ns with a pulse frequency of 10 kHz.
 3. The methodof claim 2 further including the step of focusing the Nd:YAG laser about0.5 mm above the first carbon surface.
 4. The method of claim 3 furtherincluding the step of defocusing to an about 9 μm diameter beam spotlaser output pulses of 17 μJ.
 5. The method of claim 4 wherein theimplanting occurs at 7 degrees to the C(100) axis; wherein theelectrically conducting carbon layer is about 50 nm beneath the firstcarbon surface; and wherein the electrically conducting carbon layer isabout 200 nm thick.
 6. The method of claim 5 further including the stepof ensuring no damage at the first carbon surface.
 7. The method ofclaim 6 further including the step of making electrical contact to theelectrically conducting carbon layer.
 8. A method of laser ablation forelectrical contact to a buried electrically conducting layer in diamondcomprising: polishing a single crystal diamond substrate having a firstcarbon surface; implanting the diamond with a beam of 180 KeV followedby 150 KeV C⁺ ions at fluencies of 4×10¹⁵ ions/cm² and 5×10¹⁵ ions/cm²respectively; forming an electrically conducting carbon layer beneaththe first carbon surface; utilizing a diode pumped tripled Nd:YAG laserat 355 nm wherein the laser pulse duration is about 35 ns with a pulsefrequency of 10 kHz; focusing the Nd:YAG laser about 0.5 mm above thefirst carbon surface; preventing damage at the first carbon surface; andablating the single crystal diamond which lies between the electricallyconducting layer and the first carbon surface.
 9. The method of claim 8further including the step of making electrical contact to theelectrically conducting carbon layer.
 10. A device comprising: a singlecrystal diamond substrate with a buried electrically conducting layerwith two 300×300 μm square openings spaced about 2.45 mm apart milledinto the diamond by ablating the carbon above wherein the step ofablating uses a diode pumped tripled Nd:YAG laser at 355 nm wherein theNd:YAG laser was focused about 0.5 mm above the first carbon surface thedefocusing to an about 9 μm diameter beam spot laser output pulses of 17μJ wherein the laser pulse duration is about 35 ns with a pulsefrequency of 10 kHz and wherein the square openings have electricalcontacts and wherein the resistance measured between the square openingsis dominated by the buried electrically conducting layer and on theorder of about 1 kΩ.